Re: [PATCH v3 01/10] KVM: MMU: fix decoding cache type from MTRR

Alex Williamson
2015-07-12T17:33:56Z

On Wed, 2015-05-13 at 14:42 +0800, Xiao Guangrong wrote:
> There are some bugs in current get_mtrr_type();
> 1: bit 1 of mtrr_state->enabled is corresponding bit 11 of
> IA32_MTRR_DEF_TYPE MSR which completely control MTRR's enablement
> that means other bits are ignored if it is cleared
>
> 2: the fixed MTRR ranges are controlled by bit 0 of
> mtrr_state->enabled (bit 10 of IA32_MTRR_DEF_TYPE)
>
> 3: if MTRR is disabled, UC is applied to all of physical memory rather
> than mtrr_state->def_type
>
> Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
> ---
> arch/x86/kvm/mmu.c | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)

I'm seeing a significant regression in boot performance on Intel
hardware with assigned devices that bisects back to this patch. There's
a long delay with Seabios between the version splash and execution of
option ROMs, and a _very_ long delay with OVMF before the display is
initialized. The delay is long enough that users are reporting their
previously working VM is hung with 100% CPU usage on v4.2-rc1. Thanks,

Alex

>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index b78e83f..d00cebd 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2393,19 +2393,20 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page);
> static int get_mtrr_type(struct mtrr_state_type *mtrr_state,
> u64 start, u64 end)
> {
> - int i;
> u64 base, mask;
> u8 prev_match, curr_match;
> - int num_var_ranges = KVM_NR_VAR_MTRR;
> + int i, num_var_ranges = KVM_NR_VAR_MTRR;
>
> - if (!mtrr_state->enabled)
> - return 0xFF;
> + /* MTRR is completely disabled, use UC for all of physical memory. */
> + if (!(mtrr_state->enabled & 0x2))
> + return MTRR_TYPE_UNCACHABLE;
>
> /* Make end inclusive end, instead of exclusive */
> end--;
>
> /* Look in fixed ranges. Just return the type as per start */
> - if (mtrr_state->have_fixed && (start < 0x100000)) {
> + if (mtrr_state->have_fixed && (mtrr_state->enabled & 0x1) &&
> + (start < 0x100000)) {
> int idx;
>
> if (start < 0x80000) {
> @@ -2428,9 +2429,6 @@ static int get_mtrr_type(struct mtrr_state_type *mtrr_state,
> * Look of multiple ranges matching this address and pick type
> * as per MTRR precedence
> */
> - if (!(mtrr_state->enabled & 2))
> - return mtrr_state->def_type;
> -
> prev_match = 0xFF;
> for (i = 0; i < num_var_ranges; ++i) {
> unsigned short start_state, end_state;